Pneumatic rubber tires are composed of a plurality of various rubber based components which experience varying degrees of dynamic stress during the working of the tire.
For this invention, it is desired to promote stabilization of a tire rubber component in a form of a thin layer of syndiotactic polybutadiene polymer integral with at least one rubber component.
Syndiotactic polybutadiene polymer in the form of a film (e.g. film of a polybutadiene polymer containing a high content of vinyl 1,2-structure) is well known to those having skill in the pertinent art.
In general, syndiotactic polybutadiene polymer differs from other polybutadienes (e.g. differs from cis 1,4-polybutadiene rubber) in a sense that it has a vinyl 1,2-content of at least 80 percent which may vary, for example, from about 80 percent to at least about 96 percent, a thin film of which tends to be flexible although is not considered as being elastomeric in general. Moreover, it has little to no building tack for adhering to uncured conjugated diene-based rubber compositions unless it is first blended with one or more elastomers which ordinarily required an addition of a compatibilizer and perhaps a tackifying resin to do so. Therefore, an unwanted movement of the film might occur against an uncured rubber component in some cases during the tire building and forming process unless the film is at least partially pre-cured against the rubber component to form a pre-formed composite thereof prior to the tire building process, depending somewhat upon the tire component and its location in the tire assembly.
For this invention, it is desired to apply a thin syndiotactic polybutadiene film to a tire component to promote higher stability for that component. Accordingly, it is desired that no elastomer, compatabilizing agent or tackifying resin be physically blended with the syndiotactic polybutadiene, unless used in very small amounts which does not compromise the melting point of the syndiotactic polybutadiene.
Such syndiotactic polybutadiene polymer tends to be a relatively rigid (limited flexibility) crystalline polymer with poor solubility in elastomers without addition of a compatibilizer. For this invention, as indicated above, the syndiotactic polybutadiene polymer is applied as a thin film so that it presents some flexibility and is not blended with the rubber composition of the rubber component nor a compatibilizer.
The melting point (MP) of syndiotactic polybutadiene polymer varies with the content of its 1,2-microstructure. For example, typical MP values may range from about 120° C. at about an 80 percent vinyl 1,2-content up to about 200° C. to 210° C. for about a 96 percent vinyl 1,2-content for its microstructure.
Historically, a blend of syndiotactic polybutadiene polymer and a terpolymer of ethylene, vinyl acetate and vinyl alcohol, together with a compatibilizing agent, is mentioned for use as a tire innerliner in U.S. Pat. No. 5,091,467.
A process for preparing syndiotactic polybutadiene is mentioned in U.S. Pat. Nos. 4,463,146, 5,011,896 and 6,956,093.
Rubber compounding ingredients packaged in a film of syndiotactic polybutadiene as well as rubber containing a mixture of such package and sulfur curable rubber is mentioned in U.S. Pat. No. 4,957,970.
For this invention, it is desired to use a syndiotactic polybutadiene without being mixed with and therefore in an absence of (e.g. to the exclusion of) an elastomer or a terpolymer of ethylene, vinyl acetate and vinyl alcohol, whether or not used with a compatibilizing agent, although is it desired herein that it not be used with a compatibilizer.
For this invention, it is desired that the syndiotactic polybutadiene polymer has a melt point (MP) temperature of at least 120° C., alternately from about 160° C. to about 220° C., so that it retains a significant degree of dimensional stability and thereby adds stiffness and dimensional stability and support for the associated tire component at a relatively high temperature within the tire as the tire generates internal heat when it is being dynamically worked. Various tire components are expected to operate at different temperatures within the tire as the tire is operating to generate dynamic internal temperature, with some components expected to operate at higher temperatures. Accordingly, a range of melt point (MP) temperatures are presented for the syndiotactic polybutadiene polymer with the higher melt point (MP) temperature often being preferred for the associated tire components (tire components associated in a sense of containing the integral syndiotactic polybutadiene film).
For the practice of this invention, said syndiotactic polybutadiene polymer may contain a dispersion of one or more reinforcing fillers. In order to make the syndiotactic polymer integral with the rubber component, it is co-cured with a sulfur curable rubber component. For such co-curing of the film of syndiotactic polybutadiene to a sulfur curable rubber for the tire component at the interface between the rubber component and syndiotactic polybutadiene film, the co-curing of the syndiotactic polybutadiene film may rely upon:
(A) one or more sulfur curatives contained within the syndiotactic polybutadiene polymer,
(B) one or more sulfur curatives contained within the sulfur curable rubber of the rubber component, or
(C) one or more sulfur curatives contained in each of the syndiotactic polybutadiene film and sulfur curable rubber component.
The syndiotactic polybutadiene film is made integral with the sulfur curable rubber component by co-curing the film and component together at an elevated temperature in which the film and rubber component are made integral with each other at the interface between the film and rubber component.
For the practice of this invention, the sulfur rubber component may be comprised of a composite of the syndiotactic polybutadiene film positioned on and integral with at least one of the external surfaces of a rubber composition comprised of a conjugated diene-based elastomer. In this manner, then,
(A) the composite may be a rubber component which contains a thin syndiotactic polybutadiene film positioned on and integral with one of its surfaces, or
(B) the composite may be a rubber component positioned between and integral with two syndiotactic polybutadiene films. Such rubber component may then be considered as being sandwiched between and integral with two syndiotactic polybutadiene films, or
(C) the composite may be a syndiotactic polybutadiene polymer film positioned between and integral with two rubber components. Such composite may then be considered as being a thin syndiotactic polybutadiene film sandwiched between and integral with two rubber components of the tire.
In practice, a composite of the syndiotactic polybutadiene film and rubber component may be a pre-formed, and may optionally be a partially pre-cured, composite (partially pre-cured under conditions of elevated temperature and pressure to form the composite) prior to building the composite into the tire and the resulting assembly cured in a suitable mold under conditions of elevated temperature and pressure to form the resultant tire.
A significant aspect of this invention is the promotion of dimensional stability (e.g. promote a degree of rigidity) for the tire component provided by the integral syndiotactic polybutadiene polymer film on the rubber surface which is adhered to the rubber in a sense of being co-cured therewith instead of use of an adhesive layer between the rubber and syndiotactic polybutadiene film.
Another significant aspect of the invention is the formation of an air barrier layer (e.g. oxygen barrier layer) within the tire which will provide a reduced flow of air, particularly oxygen, into the tire component which will thereby promote improved age durability for the tire itself.
Further, it is desired to not blend the syndiotactic polybutadiene film with elastomers because such addition to the syndiotactic polybutadiene is seen herein as tending to dilute, or degrade, the aforesaid dimensional stability and barrier property promotion by the integral syndiotactic polybutadiene polymer film.
In the description of this invention, the term “phr” where used means “parts of material by weight per 100 parts by weight of rubber”. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “rubber composition” and “compound” may be used interchangeably unless otherwise indicated.
The term “melting point, or MP” as used herein means the melting temperature of the syndiotactic polybutadiene polymer measured by conventional differential scanning calorimetry using a 10° C./minute temperature rise.